A dissident republican was shot dead in the pub he was running despite an armed garda patrol on a neighbouring street, it has been revealed.

One of the force's top officers, Assistant Commissioner John O'Mahoney, said the murder of Micky Barr highlighted the lengths underworld, terrorist and organised crime gangs were willing to go to.

The 35-year-old, originally from Strabane, Co Tyrone, was shot up to three times after two men, at least one of whom was armed, burst into the Sunset House bar in Dublin's Summerhill.

The Assistant Commissioner revealed an armed garda unit was around the corner when the attack was launched.

"I think it's indicative of the challenge we are facing that people are willing to go and commit a crime of this nature despite the presence or close presence of armed gardai," he said.

The three man gang - two of whom are described as skinny and wearing masks - escaped from Summerhill via Ballybough towards Drumcondra in a silver Audi with the registration 04 C 17738.

They set fire to it on Walsh Road and then made a second getaway towards Home Farm Road in a silver saloon car.

More than a dozen people were in the pub when the attack happened, including a man with special needs who had to be carried away from the scene after suffering severe shock.

Despite the brazen killing Mr O'Mahoney said officers were foiling attacks, including the attempt on a person's life one night last week.

"These are the things that are happening and they happen on a regular basis," he said.

Barr was due to be sentenced in the Special Criminal Court in Dublin on Thursday for handling stolen electrical equipment in a hotel in July 2014 where a bomb had been found in a car two months earlier.

Republican supporters described Barr as an "ex-Republican POW".

His murder was the first of two in Dublin on Monday night with speculation immediately that he had been targeted as part of the bloody underworld feud between the Kinahan and Hutch families and their associates.

But there have been conflicting reports as to whether he was the intended target or if a member of the Hutch family was in the pub at the time.

Mr O'Mahoney said both the feud and dissident links were lines of inquiry in the murder probe.

And he appealed for crime figures to end the bloody dispute that has left at least five people dead since last autumn.

"As a law enforcement officer and a policeman of course we would appeal to everybody to take stock and to see that one life after another is not going to solve anything," he said.

Gardai also appealed for local communities to help them in the fight against the upsurge in organised crime and associated vendettas.

Local Dublin city councillor Nial Ring was about 300 yards from the pub taking down 1916 posters on the street when the attackers struck.

"It was utter chaos, people out on the street," he said.

Barr was living in Ballybough, near the pub he had been running and had previous addresses in the Ballymun area and also in Finglas.

Earlier this month he pleaded guilty to handling stolen goods at Finnstown House Hotel, Newcastle Road, Lucan, Co Dublin on July 18 2014.

Two months earlier at the same hotel a bomb was found in the boot of a car.

Barr was not facing any charges in relation to that.

In November 2014, when charged with the offence, Barr was also charged with IRA membership but the charge was later dropped.

The two sides in the murderous Kinahan-Hutch feud have carried out a series of attacks in Spain and Dublin which have claimed the lives of at least five people since late last year.

The Sunset Bar is about a mile from where Martin O'Rourke, an innocent father and former drug user, was shot dead just over a week ago.

He was caught in the crossfire in Sheriff Street as another murder bid linked to the feud was launched.

That in turn was a mile or so from where taxi driver Eddie Hutch, brother of Gerry "The Monk" Hutch, was shot dead at his home off North Strand in February.

The Kinahan-Hutch feud spiralled into a killing spree when Gary Hutch was shot dead in an apartment complex near Marbella on Spain's Costa del Sol last September.

His killing is believed to have been avenged in the Regency Hotel attack, an audacious shooting spree with assault rifles in early February which claimed the life of David Byrne, an associate of the Kinahan family.

Within days, Eddie Hutch was dead, and, at the end of last month, Noel Duggan, an old friend of suspected armed robber Gerry "The Monk" Hutch, was shot dead at his home in Ratoath.

Fianna Fail pledged to raise the issue of Garda resources when talks on a minority government resume.

Sinn Fein deputy leader Mary Lou McDonald said there was upset, distress and anger on the ground following the latest murders.